Nucleating agents for thermoplastic resins

ABSTRACT

This invention relates to the use of a special class of sulfonic and phosphonic acids, and salts of the acids, to give improved nucleation for foam extrusion of thermoplastics using either dissolved gas or chemical blowing agents to produce foam.

FIELD OF THE INVENTION

This invention relates to the use of a special class of sulfonic and phosphonic acids, and salts of the acids, to give improved nucleation for foam extrusion of thermoplastics using either dissolved gas or chemical blowing agents to produce foam.

BACKGROUND OF THE INVENTION

Conductive wire is often used to transmit electronic signals. The wire must be protected, or insulated, and thermoplastic coatings are ordinarily used for this. The thermoplastic is extruded from a molten state onto and around the wire. The thermoplastic materials are selected on the basis of having a low dielectric constant and a low dissipation factor. It has previously been found that if the thermoplastic material is foamed as it is applied to the wire, the dielectric constant is desirably lowered, due to the formation of numerous small non-interconnected cells in the foam. In U.S. Pat. No. 3,072,583 is disclosed a nucleated foaming process for extruding perfluorinated polymer foam around transmission wire with a dissolved gas blowing agent. Foamed insulation using other thermoplastic materials, especially polyolefins, is also widely used.

A nucleating agent is needed in the foaming process to achieve uniform, small-diameter cell structure. A preferred art-recognized nucleating agent for fluoropolymer resins is boron nitride, an inert white ceramic powder. A 0.5-2.0 weight % loading of boron nitride, based on polymer, normally provides adequate foam cell nucleation. The lower the specific gravity of the molten resin, the more nucleant is needed, on a weight basis. Thus, an ethylene(E)/tetrafluoroethylene(TFE) copolymer(ETFE copolymer) with a melt specific gravity of about 1.3 requires more nucleant than a tetrafluoroethylene(TFE)/hexafluoropropylene(HFP) copolymer(TFE/HFP copolymer) at a melt specific gravity of 1.6.

U.S. Pat. No. 4,764,538 discloses synergistic combinations of boron nitride and certain inorganic salts which give greatly enhanced foam nucleation in fluoropolymers. This combination not only allows a significant reduction in the amount of boron nitride needed but also gives improved foam in terms of smaller cells. Improved capacitance and diameter uniformity are also attained.

The present invention provides a special class of sulfonic and phosphonic acids, and salts of the acids, which give very efficient nucleation of a wide variety of thermoplastic materials at low concentrations without the use of boron nitride or any other traditional type of nucleating agent.

As used herein the term "thermoplastic" means and is intended to include any organic material that is rendered soft and moldable by application of heat (Hackh's Chemical Dictionary, Julias Grant, 4th edition, McGraw-Hill Book Company, New York, N.Y.). Other terms used synonymously herein with "thermoplastic" and "thermoplastic material" are "thermoplastic resin" and "polymer," it being understood that the resin or polymer must be capable of being rendered soft and moldable by application of heat, that is, it is melt processible. The term "composition" as it is used herein is intended to include the thermoplastic material and other ingredients or additives which, individually, may or may not be melt processible, as will be discussed in greater detail in the Detailed Description of the Invention.

The compositions of this invention are useful in producing foams for other applications as well as for wire coating. Such examples include, but are not intended to be limited to, foamed sheet for electrical insulation, heat insulation and cushioning; foamed pipe and tubing; and injection molded pieces.

SUMMARY OF THE INVENTION

This invention resides in a foamable thermoplastic resin composition containing at least one of a special class of sulfonic and phosphonic acids, and salts of the acids, which compositions significantly enhance foam nucleation, as evidenced by smaller foam cells, higher foam void contents, and/or greater uniformity of cell size. The compositions contain 50 to 8000 ppm, preferably 250 to 3000 ppm, by weight, for most compositions, of one or more of the acids and/or salts which are of the formula

    [Z(CF.sub.2).sub.x (CF.sub.2 CFX).sub.p (R').sub.y (CH.sub.2).sub.z RO.sub.3 ].sub.n M

wherein:

Z is CCl₃, CCl₂ H, H, F, Cl or Br;

each X, independently, is selected from H, F or Cl;

R is sulfur or phosphorus;

M is H or a metallic, ammonium, substituted ammonium or quaternary ammonium cation;

x is an integer and is 0 to 10;

p is an integer and is 0 to 6;

y is 0 or 1;

z is an integer and is 0 to 10;

x+y+z+p is a positive integer or, if x+y+z+p is 0, Z is CCl₃ or CCl₂ H;

n is the valence of M; and

R' is a C₁₋₁₆ straight or branched chain perfluoroalkylene diradical containing a perfluorinated alicyclic ring; a C₁₋₆ perfluorinated aliphatic polyether diradical with repeat units selected from [CF₂ O], [CF₂ CF₂ O] and [CF₂ CF(CF₃)O]; or, when z is 0, an aromatic diradical.

By "foamable, thermoplastic resin composition" is meant any thermoplastic resin composition that is foamable by means of a gas injection process or through use of a chemical blowing agent.

Preferably, M is a metallic cation. When M is H, the resultant sulfonic or phosphonic acid may not be effective in all thermoplastic resin compositions. When M is selected so as to be other than a metallic cation or H, it preferably should be such that the resulting nucleating agent is stable at the foam processing temperature. Failure to provide such consideration in the selection of M may lead to in situ decomposition of the cationic moiety, with resultant conversion of M to H. The significance of this aspect of the invention will be readily understood in view of the significance of M on the foaming process, as described hereinafter in the Detailed Description of the Invention.

The stability of the nucleating agent at the foam processing temperature must be considered not only in connection with the selection of M but also in connection with the selection of the other symbols in the above formula. More specifically, it is to be understood that the nucleating agent should be stable at the foam processing temperature which can vary substantially depending on the thermoplastic being foamed. Thus, in the above formula, when each of x, p and y is 0, z is 1 to 10 and Z is CCl₃ or CCl₂ H, the resultant nucleating agent, for example. trichloroethane sulfonic acid or its M salt will be stable at normal polyethylene foam-processing temperatures, such as below about 315° C., but it will not be stable at the temperature needed to foam high-melting thermoplastics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides graphical data which may be used in the selection of M in the formula for the nucleating agent (a TBSA salt, defined hereinafter) in the foaming of TFE/HFP copolymer.

FIG. 2 provides graphical data which may be used in the selection of M in the formula for the nucleating agent (a TBSA salt) in the foaming of polyethylene.

DETAILED DESCRIPTION OF THE INVENTION

The thermoplastic resins useful in the present invention include all such resins which can be foamed by a gas injection process and/or by use of a chemical blowing agent. In particular, suitable thermoplastics are those with a dielectric constant of less than 18, preferably less than 12, for example, 10.2 (Example 15 herein). Especially suitable for making foamed electrical insulation by this invention are those resins with a dielectric constant of less than 3. Following is a list of thermoplastic materials, most of which are commercially available, and their dielectric constants relative to air having a dielectric constant of 1.0:

    ______________________________________                                         PTFE or TFE/HFP Copolymer                                                                               2.0                                                   TFE/PAVE Copolymer       2.1                                                   Polypropylene            2.25                                                  Polyallomers             2.25                                                  Polyethylene             2.3                                                   ETFE Copolymer           2.6                                                   E/CTFE Copolymer         2.6                                                   EVA Copolymer            2.75-2.8                                              Polycarbonate            2.96                                                  Polyimide                3.0-3.5                                               Polysulfone              3.13                                                  PVC                      4.5-5.8                                               Nylon                    4.5                                                   PVF.sub.2                2.98-8.4                                              Urethanes                6.7-7.5                                               Neoprene @ 1000 KM       16-17                                                 ______________________________________                                    

Preferred examples of suitable thermoplastics include melt processible fluoropolymers; polyolefins; polyamides; polyesters; vinyl polymers, such as polyvinyl chloride; uncured natural and synthetic rubbers; and the broad class of resins known as thermoplastic elastomers. It is to be further understood that the thermoplastic resins which are useful herein include blends of polymers and blends of polymers with non-polymeric materials, for example, pigments, fire retardants, minerals and carbon, that is, any material commonly present in foamed compositions. In other words, the invention is useful with any virgin or blended resin that is capable of being foamed by a gas injection process or by means of a chemical blowing agent.

Foamed fluoropolymer wire insulation, one of the preferred embodiments of this invention, is generally produced in the art by a continuous, gas injection process using chlorodifluoromethane(F-22), nitrogen, carbon dioxide, or other gas mixture which is soluble in the molten resin, using an extruder screw designed for foam operations, and using boron nitride as a nucleating agent. In such processes the gas dissolves in the molten resin inside the extruder. Absorption follows Henry's Gas Law which states that the equilibrium value of the mole fraction of gas dissolved in a liquid is directly proportional to the partial pressure of the gas above the liquid surface. By adjusting the gas pressure in the extruder, the amount of gas dissolved in the melt can be controlled. Generally, the greater the amount of gas dissolved in the melt, the greater the void volume in the resulting foam.

Chlorodifluoromethane(F-22) is especially suited for foaming because it easily dissolves in molten fluoropolymer resins, and its thermal stability is adequate to survive the holdup time in the extruder when mixed with molten resin at temperatures up to 380° C.

As an alternative process, a chemical blowing agent (a chemical which is unstable at the polymer processing temperature and liberates a gas, such as nitrogen, carbon dioxide or hydrogen) can be incorporated into the thermoplastic to provide the dissolved gas which causes bubble formation.

Foam cell formation actually starts shortly after the molten resin containing the blowing agent passes out of the extrusion die. The gas dissolved in the molten resin comes out of solution because of the sudden drop in melt pressure as the extrudate exits the extrusion die. As shown in U.S. Pat. No. 4,764,538, extrusion speed and drawdown influence foaming. The polymer is solidified when the extrudate enters water in a cooling trough, stopping foam cell growth.

A nucleating agent is needed to achieve uniform, small-diameter cell structures. The preferred commonly used nucleating agent for fluoropolymer resins has been boron nitride, an inert white ceramic powder. The present invention covers a special family of sulfonic and phosphonic acids, and salts of the acids, which give very efficient nucleation at low concentrations. The optimum nucleating agent and concentration depend on the specific polymer and conditions employed in the foaming process.

The nucleating agent of this invention can be blended with virgin or blended polymers, as powder or pellets, and extruded to give a homogeneous composition containing the desired amount of the nucleating agent, generally 50-8000 ppm, preferably 250-3000 ppm. Alternatively, the agent can be added to the virgin or blended polymer as a concentrate which may contain 5-20 times the desired amount of nucleating agent, and then the concentrate is mixed with 20-5 parts of virgin or blended polymer before the foaming step.

With chemically blown compositions the blowing agent and nucleating agent can be blended with virgin or blended polymer powder and either compacted into pellets at room temperature to avoid premature decomposition of the blowing agent, or extrusion-compounded at a temperature at which the blowing agent is not decomposed to a substantial extent.

FIG. 1 and Examples 2 and 3 show how routine experimentation can help select the best cation for foaming a particular resin (TFE/HFP copolymer) with a room temperature dielectric constant of 2.05 with CHF₂ Cl using a salt of Telomer B sulfonic acid(TBSA). Telomer B sulfonic acid is of the formula F(CF₂)_(n) CH₂ CH₂ SO₃ H wherein n is a mixture of 6, 8, 10 and possibly 12, predominantly 8. The results for different cations are plotted on the FIG. 1 graph using the atomic radius of the cation as abscissa and cation valence as ordinate; above the point for each cation is shown the minimum average diameter of the foam cells in micrometers for the optimum concentration tested. It is desirable to have very small cells and high foam void content. It can be seen in FIG. 1 that the cations giving cell diameters equal to or less than about 200 micrometers fall between the two outer lines (the preferred range). The cations giving still smaller cell diameters, equal to or less than about 75 micrometers, fall between the two inner lines (optimum range). As is apparent, for TFE/HFP copolymer the best cations are Zr⁺⁺⁺⁺, Fe⁺⁺⁺, and Ba.sup. ++. The preferred nucleating agent concentration for these cations with this copolymer is 500-2000 ppm. Thus, FIG. 1 provides a means for predicting how other salts of TBSA will perform in TFE/HFP copolymers with CHF₂ Cl as blowing agent. Similar performance is observed when the thermoplastic resin is a copolymer of tetrafluoroethylene and a perfluoro(alkyl vinyl ether), such as perfluoro(propyl vinyl ether).

Table 1 which follows shows the great difference in nucleating efficiency of various bivalent salts of TBSA in the experiments depicted in FIG. 1. It also shows that efficient nucleation results in both high void content in the foam and small diameters of the foam cells.

In Table 1 and in all the remaining tables herein, the following abbreviations are used:

    ______________________________________                                         SPC:        Specific Void Content                                              AFCD:       Average Foam Cell Diameter (at mid-wall)                           M:          Micrometers                                                        % V:        % Voids in Foam as Calculated from                                             the Measured Dielectric Constant                                   cc/cm.sup.3 :                                                                              Cell Count/cm.sup.3 at the Specific Void                                       Content                                                            Conc.:      Concentration                                                      Ex St:      Extrusion Stability                                                DC:         Dielectric Constant                                                FCC:        Foam Core Capacitance (lowest value)                               pF/m:       Picofarads/meter                                                   C:          Capacitance                                                        ______________________________________                                    

                  TABLE 1                                                          ______________________________________                                         TBSA                             AFCD                                          Salt       cc/cm.sup.3    % V    (M)                                           ______________________________________                                         Barium     2,300,000      55     75                                            Strontium  763,000        59     100                                           Calcium    72,200         41     220                                           Zinc       24,800         20     508                                           ______________________________________                                    

FIG. 2 and Example 11 show a similar plot for a polyethylene which has a room temperature dielectric constant of 2.3. Salts of the same TBSA (as in FIG. 1 and Examples 2 and 3) were used, with nitrogen as the blowing agent. FIG. 2 has the same general appearance as FIG. 1, but a narrower optimum range has been chosen (cell diameters equal to or less than about 50 micrometers), and the polymer and blowing agent are different. FIG. 2 shows that with this polyethylene the preferred TBSA salts are those of Cr⁺⁺⁺, Fe⁺⁺⁺, ad Ca⁺⁺. The preferred nucleating agent concentration for these cations with this polymer is 1000-3000 ppm.

One might expect that fluorine-free salts, such as sodium lauryl sulfate, would be effective nucleating agents for polyethylene, just as the fluorine-containing salts of this invention are for fluoropolymers. Surprisingly, this is not the case, as shown by Comparative Example B between Examples 11 and 12 herein. Comparative Examples A and B show, also, the inoperability of other compounds as nucleating agents.

Also demonstrated in the examples herein are foamed compositions comprising ethylene/tetrafluoroethylene copolymers. Preferred nucleating agents for such resins include the calcium salt of Telomer B sulfonic acid and the potassium salt of a perfluorinated alkane sulfonic acid of 1-10 carbon atoms. A preferred concentration of such nucleating agents, based on the composition, is 1000 to 3000 ppm.

The broad applicability of the invention is further demonstrated in the examples using polyolefins as the thermoplastic resin. Examples of polyolefins include, but are not limited to, polyethylene, linear and branched, low density and high density. Blends of polyethylene, and blends of polyethylene or polyethylenes with other thermoplastic resins are operable herein. Preferred nucleating agents with polyethylene-containing thermoplastic resins are the potassium salt of perfluorooctane sulfonic acid, and the calcium, iron or chromium salt of TBSA, at a preferred concentration of 1000 to 3000 ppm, based on the composition.

In general, polymers having high dielectric constants respond best to TBSA salts with small cation radius and low valence. As the acid strength and/or chain length of the salt anion change in any given polymer, the optimum cation also tends to change. In the polyethylene of Example 11 the optimum cation for a TBSA salt is chromium, whereas with a perfluorooctanesulfonic acid the optimum cation is potassium; with trifluoromethanesulfonic acid, sodium is the best cation of those tested. Thus, as the strength of the sulfonic acid increases and/or chain length decreases, the optimum cation moves down or to the left in a plot, such as in FIG. 2. The phosphonic acids, and their salts, of this invention give results similar to those described above for the sulfonic acids and their salts. It is to be understood that routine experimentation may be required to find the optimum cation and anion for each thermoplastic, with dependence also on variables such as the molecular weight distribution of the thermoplastic, the polarity of the gas used, and the extrusion temperature. However, as demonstrated herein, efficient nucleation is achievable with the nucleating agents of the invention in a wide variety of polymers.

The present invention provides means for producing foamed thermoplastics having a combination of a uniformly small cell size, less than 400 micrometers, and a very high void content, greater than 50%. For example, using TFE/HFP copolymer thermoplastics, cell sizes of less than 100 micrometers (average) and void contents of greater than 60% can be achieved. Using polyethylene thermoplastics, cell sizes of less than 100 micrometers (average) and void contents of greater than 50% can be achieved. Foams of similar structure (as with polyethylenes) can also be produced using E/TFE copolymers as the thermoplastic.

EXPERIMENTAL DETAILS

In the examples herein the foaming process was carried out by either a continuous gas injection foaming process or by extruding a composition containing a chemical blowing agent in the same extruder, with no gas injection.

The continuous gas injection foaming process was normally carried out using chlorodifluoromethane (F-22) or nitrogen as the blowing agent. Other known blowing agents could be used to give substantially the same results. The foam void level was controlled by the pressure of the blowing agent. The extruder screw design allowed for gas injection and had a mixing torpedo (as disclosed in U.S. Pat. No. 3,006,029) to provide a uniform melt. This mixing torpedo allowed each resin cube blend to become a uniform melt mixture while traversing the length of the screw.

Nucleated compositions for the gas injection process can be prepared by cube blending of the resin (melt cut or strand cut) with a nucleating agent concentrate prepared by blending the sulfonic or phosphonic acid, or its salt, in the form of a powder, with resin powder and/or cubes, and extruding on a 28 mm Warner & Pfeiderer (W&P) twin screw extruder with a strand cutter to pelletize. The resulting cube blends were fed directly to the wire coating extruder hopper.

ETFE copolymer compositions containing chemical blowing agents were prepared by blending the chemical blowing agent and nucleating agent with polymer powder and compacting into cylindrical pellets about 3 mm in diameter and 6 mm in length with a lab model California Pellet Mill. These pellets were fed directly to the wire coating extruder hopper.

Foam cell size was determined at mid-wall of the foam by visual examination with a microscope or by taking a picture of a thin section of foamed wire insulation at 20-30X magnification and visually determining average cell size. In some examples the number of cells/cm³ is also given. This value is calculated from the foam void level and the average cell diameter.

Additional abbreviations used in the examples are shown in Table 2. In reciting specific polymer compositions in the table, there is no intent to limit the scope of the polymers operable herein to those specific compositions. Typical conditions used in the examples that follow are shown in Table 3. Regarding Table 3, in Column C a Maillefer extruder was used to produce the foam and an Entwistle extruder was used to produce the skin. In Column H the extruder was a Davis-Standard. In all other columns the extruder was an Entwistle. Further regarding Table 3, in Column A, a B&H 75 crosshead was used; in all other columns the crosshead was a Maillefer 4/6F. Finally, the Column C conditions were carried out to produce a foam-skin construction, using two extruders and a single crosshead.

Foam cell size depends not only on the nucleating agent, but also on other variables, such as wire speed and, in the case of melt draw extrusion, cone length (the distance from the die exit to the point where the molten tubular polymer forms on the wire), as explained in U.S. Pat. No. 4,764,538. Cone length is related to the applied vacuum inside the cone.

                  TABLE 2                                                          ______________________________________                                         TFE/HFP copolymer                                                                            a copolymer of 12.3 wt % HFP and                                               the balance TFE with a melt flow                                               rate of 6.6 at 372° C. as                                               measured by ASTM D-2116-83                                       PAVE          perfluoro(alkyl vinyl ether)                                     EVA           Ethylene/Vinyl Acetate                                           PVF.sub.2     Poly(vinylidene fluoride)                                        boron nitride SHP-325 grade from Carborundum                                                 Corp., having a BET surface area                                               of 8.6 m.sup.2 /g                                                polyarylate   aromatic polyester of Bisphenol                                                A and isophthalic acid, with a                                                 melt viscosity at 330° C. of 1600                                       Pa · s at 100 sec.sup.-1 and 2800 Pa · s                     at 10 sec.sup.-1                                                 E/TFE         copolymer of 19.6 wt % ethylene,                                               2.2 wt % perfluorobutyl                                                        ethylene, and the balance TFE,                                                 with a melt flow rate of 14 at                                                 297° C. by ASTM D-3159-83                                 Ficel ®   Ficel ® AF-100 formulated zinc                                             compound, a chemical blowing                                                   agent from Sherex Chemical Co.                                   Expandex ®                                                                               Expandex ® 175, barium salt of                                             5-phenyl tetrazole, a chemical                                                 blowing agent of Olin Chemicals,                                               Stamford, Connecticut                                            TFE/PPVE(1)   copolymer of TFE and 3.9 wt. %                                   copolymer     perfluoro(propyl vinyl ether),                                                 with a melt viscosity of 2.2 ×                                           10.sup.4 poise at 372° C.                                 TFE/PPVE(2)   copolymer of TFE and 3.9 wt. %                                   copolymer     perfluoro(propyl vinyl ether),                                                 with a melt viscosity of 4.25 ×                                          10.sup.4 poise at 372° C.                                 E/CTFE copolymer                                                                             copolymer of ethylene and                                                      chlorotrifluoroethylene                                          FEP copolymer copolymer of TFE and 12.3 wt. %                                                hexafluoropropylene, with a melt                                               flow rate of 6.6 at 372° C. (ASTM                                       D-2116-83)                                                       ZrS-10        zirconium (+4) salt of TBSA                                      CrS-10        chromium (+3) salt of TBSA                                       CeS-10        cerium (+4) salt of TBSA                                         KS-10         potassium salt of TBSA                                           HS-10         TBSA                                                             AS-10         aluminum salt of TBSA                                            SrS-10        strontium salt of TBSA                                           CaS-10        calcium salt of TBSA                                             ZnS-10        zinc salt of TBSA                                                BaS-10        barium salt of TBSA                                              LS-10         lithium salt of TBSA                                             FS-10         iron (+3) salt of TBSA                                           BS-6A         barium p-(perfluoro[1,3-                                                       dimethylbutyl]) benzene                                                        sulfonate                                                        BS-9A         barium p-(perfluoro[1,3,5-                                                     trimethylhexyl]) benzene                                                       sulfonate                                                        BS-8          barium perfluorooctane sulfonate                                 KS-8          potassium perfluorooctane                                                      sulfonate                                                        KS-8C         potassium perfluorocyclo-                                                      hexylethane sulfonate                                            NaS-1         sodium trifluoromethane                                                        sulfonate                                                        KS-1          potassium trifluoromethane                                                     sulfonate                                                        KS-1(H)       potassium methane sulfonate                                      NaTCA         sodium trichloroacetate                                          BTBP          barium salt of F(CF.sub.2).sub.n CH.sub.2 CH.sub.2 PO.sub.3                    H                                                                              wherein n is a mixture of 6, 8,                                                10 and possibly 12,                                                            predominantly 8                                                  NTBP          sodium salt of F(CF.sub.2).sub.n CH.sub.2 CH.sub.2 PO.sub.3                    H                                                                              wherein n is a mixture of 6, 8,                                                10 and possibly 12,                                                            predominantly 8                                                  LL1121B       barium perfluoro(2,5-                                                          dimethyl)-3,6-dioxatridecanoate                                  BC14(06)      barium perfluoro 3,5,7,9,11,13-                                                hexaoxatetradecanoate                                            BS-12(H)      barium lauryl sulfate                                            NS-12(H)      sodium lauryl sulfate                                            CC-18(H)      calcium stearate                                                 BaC-8         barium perfluorooctanoate                                        BaC-9         barium perfluorononanoate                                        AWG-26        Solid copper wire 404                                                          micrometers in diameter                                          AWG-24        Solid copper wire 510                                                          micrometers in diameter                                          AWG-22        Solid copper wire 635                                                          micrometers in diameter                                          ______________________________________                                    

                                      TABLE 3                                      __________________________________________________________________________               A       B       C            D       E                               __________________________________________________________________________     Type Foaming                                                                             Gas Injec-                                                                             Gas Injec-                                                                             Gas Injection,                                                                              Gas Injec-                                                                             Chemical                                  tion, F-22                                                                             tion, F-22                                                                             F-22         tion, F-22                                                                             Blow                            Extruder                                                                       Diameter (mm)                                                                            31.75   31.75   60 mm  25.4  31.75   31.75                           L/D Ratio 30/1    30/1    30/1   20/1  30/1    30/1                            Type                                                                           Extrusion Melt Draw                                                                              Pressure                                                                               Pressure     Pressure                                                                               Pressure                        Screw Design                                                                   Zone                                                                           Feed      4.06/10 3.81/10 4.45/10                                                                               4.57/12                                                                              3.81/10 6.6/13                          Metering  1.02/4  0.86/3.7                                                                               .97/4.0                                                                               --    0.86/3.7                                                                               --                              Gas       3.56/4  3.42/3.25                                                                              5.1/3.5                                                                               --    3.42/3.25                                                                              --                              Pumping   1.57/5  1.32/4.25                                                                              1.47/6.0                                                                              1.9/4 1.32/4.25                                                                              2.54/8                          Mixing    3.81/5.5                                                                               5.96/3.5                                                                               7.1/4.0                                                                               3.6/3.7                                                                              5.96/3.5                                                                               3.81/5.5                        __________________________________________________________________________               F       G       H            I       J                               __________________________________________________________________________     Type Foaming                                                                             Gas Injec-                                                                             Chemical                                                                               Chemical     Gas Injec-                                                                             Gas Injec-                                tion, N.sub.2                                                                          Blow    Blow         tion, N.sub.2                                                                          tion, N.sub.2                   Extruder                                                                       Diameter (mm)                                                                            31.75   31.75   50.8 mm      31.75   31.75                           L/D Ratio 30/1    30/1    28/1         30/1    30/1                            Type                                                                           Extrusion Pressure                                                                               Pressure                                                                               Pressure     Pressure                                                                               Pressure                        Screw Design                                                                   Zone                                                                           Feed      3.81/10 6.6/13  8.25         3.81/10 4.06/10                         Metering  0.86/3.7        Note: long   0.86/3.7                                                                               1.02/4                                                    transition                                           Gas       3.42/3.25                    3.42/4.25                                                                              3.56/4                          Pumping   1.32/4.25                                                                              2.54/8  6.6/10       1.32/4.25                                                                              1.57/5                          Mixing    5.96/3.5                                                                               3.81/5.5             5.96/3.5                                                                               3.81/5.5                        __________________________________________________________________________               A    B   C      D   E   F   G   H   I   J                            __________________________________________________________________________     Temperatures (°C.)                                                      Rear      360  315 329 304                                                                               332 250 260 160 326 299 165                          Center Rear                                                                              382  315 315    260 285 232 188 332 299 177                          Center    371  315        260 285 232 201 332 291 165                          Center Front                                                                             382  315        268 320 254 201 321 288 168                          Front     382  315 321 315                                                                               273 330 273 201 321 282 171                          Adaptor   371  315 315 315                                                                               223 315 273 204 357 279 182                          X Head    326  325 327    282 320 288 204 360 282 177                          Die       326  330 329    282 320 288 204 371 282 177                          Melt      387  320 343    125 285 278 210 343 282 178                          Screw Speed                                                                              80   60  41     72  6   80  70  5   40  25                           Air Gap                                                                        (meters)  4.5  4.6 10     4.6 4.6 4.6 4.6 10  4.6 4.5                          Wire Size (mm)                                                                           0.643                                                                               0.51                                                                               0.51   0.58                                                                               0.643                                                                              0.508                                                                              0.51                                                                               0.406                                                                              0.508                                                                              0.51                         Wire Speed                                                                     (m/min.)  18   41  838    22  34  48.6                                                                               43  30  20  7.6                          Wire Preheat                                                                   (°C.)                                                                             65   132 223    104 65  99  82  150 93  none                         Crosshead Melt                                                                 Pressure                                                                       (MPa)     14(FEP)                                                                             4.9 13.8   4.9 11.7                                                                               9.6 23.4                                                                               4.8 5.2 5.2                          Gas Pressure                                                                   (IKPa)    0.7  0.7 0.5    0.7 None                                                                               2.1 None                                                                               None                                                                               2.4 2.1                          Vacuum(mm Hg)                                                                            380  None                                                                               None   None                                                                               None                                                                               None                                                                               None                                                                               None                                                                               None                                                                               None                         Die (mm)  5.05 1.32                                                                               0.73   1.32                                                                               0.914                                                                              1.3 1.3 0.56                                                                               1.6 2.1                          Guide Tip (mm)                                                                           1.91 0.59                                                                               0.56   0.59                                                                               0.685                                                                              0.64                                                                               0.56                                                                               0.43                                                                               0.58                                                                               0.56                         Foam Core Size                                                                 (mm)      3.30 2.03                                                                               0.83   1.45                                                                               1.15                                                                               2.15                                                                               2.3 0.7 2.28                                                                               ˜2.5                                             to                                                                             1.9                                                  __________________________________________________________________________      Note: Balance of Extruder screw 1/d lengths are transitions and the screw      front tip.                                                               

EXAMPLE 1

A. TFE/HFP copolymer was cube blended with a 5 weight percent boron nitride concentrate (compounded with TFE/HFP copolymer in a 28 mm W&P twin screw extruder as described earlier) in the ratio needed to give a product with 0.25% boron nitride.

B. A second composition was prepared from TFE/HFP copolymer by blending a 5 weight % boron nitride concentrate plus a 0.3 weight % KS-10 concentrate to give 0.25% boron nitride and 330 ppm KS-10.

C. A third composition was prepared from TFE/HFP copolymer by blending with a 5 weight percent boron nitride concentrate plus a concentrate containing 0.3% calcium tetraborate to give 0.25% boron nitride and 125 ppm calcium tetraborate.

D. A fourth composition was prepared from TFE/HFP copolymer by blending with a 5 weight % boron nitride concentrate plus a concentrate containing 0.3% calcium tetraborate and a 0.3 weight % TBSA concentrate to give 0.25% boron nitride and 125 ppm calcium tetraborate plus 330 ppm of TBSA.

These four compositions were extruded onto wire under the conditions of Table 3, Column A with the following results:

                  TABLE 4                                                          ______________________________________                                                                AFCD                                                    Composition % V        (M)     cc/cm.sup.3                                     ______________________________________                                         A.          55         381     18,000                                          B.          60         152     312,000                                         C.          55         100     2,800,000                                       D.          58         76      12,000,000                                      ______________________________________                                    

As seen from these results, the KS-10 can be used with boron nitride, and it gives enhanced foam nucleation. Similarly, TBSA can be used in conjunction with boron nitride and calcium tetraborate, and it gives enhanced nucleation.

EXAMPLE 2

TFE/HFP copolymer was blended with a 5% boron nitride concentrate and with a 0.3% BaS-10 concentrate to give the compositions shown in Table 5. Each composition was foamed onto wire using the conditions of Table 3, Column A.

                  TABLE 5                                                          ______________________________________                                                          Conc.   AFCD                                                  Additive         (ppm)   (M)                                                   ______________________________________                                         None             --      635                                                   Boron nitride    5000    125                                                   BaS-10           125     760                                                   BaS-10           500     100                                                   BaS-10           1000    75                                                    BaS-10           1500    255                                                   ______________________________________                                    

As shown above, BaS-10 is a very effective nucleant at low concentrations compared with boron nitride, a traditional nucleating agent used with TFE/HFP copolymers. It should also be noted that there is an optimum concentration for the BaS-10 nucleating agent.

EXAMPLE 3

Samples of TFE/HFP copolymer containing several concentrations of various salts of TBSA were prepared by blending virgin resin with the appropriate ratio of concentrate containing 0.3% of the fluorosulfonic acid salt. These compositions were foamed onto wire as described in Table 3, Column A, with the results shown in Table 6. The last column shows the deviation from the average foam core diameter of 3300 micrometers.

                  TABLE 6                                                          ______________________________________                                                   Conc.   AFCD            Deviation                                    Additive  (ppm)   (M)        % V  (M)                                          ______________________________________                                         Boron                                                                          nitride                                                                        (control) 5500    127        58   ± 76                                      ZrS-10    75      685        61   ± 127                                               300     387        58   ± 127                                               500     76         67   ± 76                                                1000    76         65   ± 76                                                2000    203        65   ± 127                                               3000    203        56   ± 127                                     CrS-10    75      635        52   ± 50                                                300     431        59   ± 76                                                500     101        62   ± 50                                                1500    127        62   ± 50                                                2000    89(a)      37   --                                                     3000    (b)        (b)  --                                           CeS-10    63      559        63   ± 127                                               250     635        65   ± 127                                               500     762        72   ± 127                                               1000    381        66   ± 76                                                2000    178        67   ± 76                                                3000    127        62   ± 50                                      KS-10     63      762        78   ± 127                                               250     304        65   ± 76                                                500     241        66   ± 76                                                1000    228        64   ± 76                                                2000    (b)        (b)  --                                           HS-10     63      762        61   ± 76                                                250     203        65   ± 76                                                500     127        64   ± 76                                                1000    127        61   ± 76                                      AS-10     500     889        60   ± 100                                               1000    127        58   ± 50                                                2000    139        60   (b)                                          SrS-10    500     139        58   ± 100                                               2000    127        59   ± 75                                                3000    228        57   ± 127                                     CaS-10    500     203        41   ± 76                                                1000    228        41   ± 100                                               1500    406        41   ± 127                                     Zn-10     1000    508        20   ± 127                                               2000    381        20   ± 127                                               3000    381        20   ± 127                                     BaS-10    125     762        20   (c)                                                    500     101        55   ± 50                                                1000    76         55   ± 50                                                1500    254        40   ± 150                                     LS-10     500     178*       55   ± 75                                                1000    178*       56   ± 75                                      FS-10     75      406        52   ± 250                                               150     101        55   ± 50                                                300     101        58   ± 50                                                500     76         55   ± 50                                                1000    76         55   ± 59                                      ______________________________________                                          *Cells collapsed to ellipses to the indicated minor diameter. If these         cells were spherical cells, they would be 280 micrometers in diameter.         (a) open cells                                                                 (b) gas injector probe plugged                                                 (c) no record                                                            

As seen above, all of these fluorosulfonic acid salts (and the HS-10 itself) function as nucleating agents. Some are much better than others and the performance of each one varies with concentration. In addition to cell size and void content variation, there are significant fluctuations in diameter and capacitance.

EXAMPLE 4

A series of samples containing various levels of salts of three other types of sulfonic acids and a phosphonic acid were also prepared, extended in TFE/HFP copolymer and foamed onto wire, as described in Example 3, with the results shown in Table 7. The last column shows the deviation from the average core diameter of 3,300 micrometers.

                  TABLE 7                                                          ______________________________________                                                  Conc.     AFCD            Deviation                                   Additive (ppm)     (M)     % V     (M)                                         ______________________________________                                         BS-9A     500      762     75      (b)                                                  1000      762     62      (b)                                                  2000      381     60      ± 75                                              3000      253     54      ± 75                                     BS-8      500      254     56      ± 100                                             1000      228     57      ± 75                                              2000      101     59      ± 50                                     KS-8C    1500      635     40      ± 200                                             3000      400     40      ± 200                                    BTBP     1500       64     80      ± 200                                             3000      25-67   ˜70(a)                                                                           (c)                                         ______________________________________                                          (a) open cells                                                                 (b) unstable; diameter control was poor                                        (c) unstable; foaming in crosshead                                       

As seen from the above results, the three barium salts are effective as nucleating agents for TFE/HFP copolymers, whereas the potassium salt is not.

Comparative Example A

In contrast to Examples 3 and 4, samples containing various levels of other types of salts (not within the invention) in TFE/HFP copolymers were prepared in the same manner from concentrates containing 0.3% of the salt. These compositions were also foamed onto wire as described in Table 3, Column A with the results shown in Table 8.

                  TABLE 8                                                          ______________________________________                                                    Conc.        AFCD                                                   Additives  (ppm)        (M)     % V                                            ______________________________________                                         LL1121B     500         1016    65                                                        1000         558     46                                                        2000         1016    45                                                        3000         1016    63                                             BC14(06)    500         890     52                                                        1000         762     55                                                        2000         890     55                                                        3000         1016    62                                             BS-12(H)    500         431     57                                                        1000         711     52                                                        2000         889     57                                                        3000         889     57                                             CC-18(H)    500         685     60                                                        1000         1270    62                                                        2000         1524*   63                                             BaC-8       500         635     46                                                        1000         635     45                                             BaC-9       500         1016    42                                                        1000         762     53                                                        1500         889     53                                             ______________________________________                                          *Foam cells blistered outside the insulation surface. All of the               compositions gave large cell size (i.e., greater than 400 micrometers) an      produced unstable capacitance and diameter in extrusion.                 

EXAMPLE 5

Compositions with tetrafluoroethylene(TFE)/perfluoro(propyl vinyl ether) (PPVE) copolymers (TFE/PPVE copolymers) were made up using a 0.3% concentrate of BaS-10 as described in Example 2. These compositions were foamed onto wire according to the conditions of Table 3, Column A. The results are shown in Table 9. The capacitance and diameter stability during extrusion are shown in the last column.

                  TABLE 9                                                          ______________________________________                                                     Conc.   AFCD                                                       Additive    (ppm)   (M)       % V  Remarks                                     ______________________________________                                         TFE/PPVE(1)                                                                    Boron nitride                                                                              5000    300       55   (a)                                         (control)                                                                      BaS-10       63     406       51   (b)                                                      125    330       57   (b)                                                      250    304       55   (b)                                                      500    304       55   (b)                                                     1000    102       55   (b)                                                     2000     89       57   (b)                                                     3000    127       54   (b)                                         TFE/PPVE(2)                                                                    Boron nitride                                                                              5000    255       55   (a)                                         (control)                                                                      BaS-10       500    286       57   (c)                                                     1000    191       58   (c)                                                     2500     51       60   (a)                                                     3000     76       62   (a)                                         ______________________________________                                          (a) Steady extrusion                                                           (b) Head pressure very low in extrusion and some foaming in the crosshead      occurred to make stability of the process borderline. This occurred            because the extruder screw design was not optimized for TFE/PPVE(1).           (c) Extrusion not steady                                                 

As seen from these results, BaS-10 is an effective nucleating agent for TFE/PPVE copolymer resins. Note that there is an optimum concentration range to achieve the smallest cell size.

EXAMPLE 6

ETFE copolymer containing 2 wt.% boron nitride or varying amounts of fluorosulfonic acid salts as indicated in Table 10 was foamed onto wire at about 40 meters/minute according to the conditions of Table 3, Column B. This produced a 0.7 mm thick foamed insulation on AWG-24 solid copper wire having capacitance values near 60 pF/m at a foam void level of about 60% (Table 10 except for KS-8 and KS-8C). For KS-8 and KS-8C this produced a 0.28 mm thick foamed insulation on AWG-24 wire having capacitance values near 136 pF/m (a foam void level of about 42%). Relative foaming performance, as indicated by insulation diameter and capacitance variability and by foam cell size, is shown in Table 10. The last column shows the deviation from the average core diameter of 1900 micrometers and the deviation from the average capacitance of 62-69 pF/m, except for KS-8 and KS-8C wherein the deviations are from an average core diameter of 1068 micrometers and an average capacitance of 138 pF/m.

                  TABLE 10                                                         ______________________________________                                                  Conc.     AFCD        Deviation                                       Additive (ppm)     (M)         (M; pF/m)                                       ______________________________________                                         None     None      432           (b)                                           LS-10    750       --            (a)                                                    1500      --            (a)                                                    3000      --            (a)                                           CaS-10   750       76                                                                   1500      51            (c) ± 13; ± 0.3*                                 3000      51                                                          BaS-10   750       178                                                                  1500      178           (c) ± 38; ± 1.5                                  3000      178                                                         ZnS-10   750       432                                                                  1500      432           (d) ± 40; ≧ 15                               3000      432                                                         AS-10    750       500                                                                  1500      432           (d) ± 50; ≧ 15                               3000      483                                                         FS-10    750       635                                                                  1500      635           (c) ± l0; ± 0.3                                  3000      500                                                         KS-8     250       127                                                                  500       114                                                                  1000      76            (c) ± 10; ± 0.3                                  2000      50                                                                   3000      89                                                          KS-8C    250       178                                                                  500       152           (c) ± 10; ± 0.3                                  2000      64                                                                   3000      38-76                                                       ______________________________________                                          (a) gas injector probe plugged                                                 (b) low void content, 32% unstable extrusion                                   (c) extrusion foaming stable                                                   (d)extrusion foaming unsteady                                                  *capacitance average of 57 pF/m                                          

EXAMPLE 7

BaS-10 was evaluated as a nucleating agent for ETFE copolymer in a high speed extrusion (625 m/min.) of thin wall foam-skin insulation on AWG-24 solid copper wire. Conditions used are given in Table 3, Column C and the results are given in Table 11. A dielectric constant of 1.7 corresponds to a foam void level of 48%.

                  TABLE 11                                                         ______________________________________                                                 C         Diameter  Foam   AFCD                                        Additive                                                                               (pF/m)    (M)       DC     (M)   Ex St                                 ______________________________________                                         2% BN   121.4 ± 1.0                                                                           1092 ± 50                                                                             1.68   50    (a)                                   (Control                                                                       Resin)                                                                         500 ppm 132.2 ± 0.3                                                                           1049 ± 13                                                                             1.71   50    (b)                                   BaS-10                                                                         1000 ppm                                                                               125.3 ± 0.3                                                                           1054 ± 13                                                                             1.63   38    (c)                                   BaS-10                                                                         2000 ppm                                                                               131.2 ± 0.3                                                                           1049 ± 13                                                                             1.70   25    (b)                                   BaS-10                                                                         3000 ppm                                                                               128.9 ± 1.6                                                                           1049 ± 25                                                                             1.68   50    (b)                                   ______________________________________                                          (a) Good, steady                                                               (b) A few capacitance spikes. These positive spikes in the capacitance         chart record occurred every 5-8 minutes.                                       (c)Excellent, very steady                                                

As seen from the above results, BaS-10 not only gives small cell size but it also improves both diameter and capacitance control in this miniature foam.

EXAMPLE 8

An E/CTFE copolymer (Halar® 500 available from Ausimont Chemical Co.) was foamed with CHF₂ Cl gas injection using a start-stop extrusion procedure. This technique was necessary to avoid immediate plugging of the gas injection probe (improper screw design). The foaming process was carried out according to the conditions given in Table 3, Column D. Nucleated compositions were prepared using a 0.3% concentrate of the salt in the copolymer. The results are given in Table 12.

                  TABLE 12                                                         ______________________________________                                                Conc.   FCC     Foam        AFCD                                        Additive                                                                              (ppm)   (pF/m)  DC    % V   (M)   cc/cm.sup.3                           ______________________________________                                         None   0       98.4    2.01  28    304   336,000                               CaS-10 1500    96.8    2.20  20    203   800,000                               CaS-10 3000    75.5    1.78  43    102   15,000,000                            ______________________________________                                    

As seen from the results, CaS-10 is a very effective foam nucleant.

EXAMPLE 9

HAP-E/TFE, a chemical-blown ETFE copolymer foam resin commercially available from Chromatics, Inc. (Bethel, Conn.) was foamed onto wire with and without added BaS-10 using the extrusion setup shown in Table 3, Column E. In each case, the HAP-E/TFE (210 grade) was blended into a 50/50 mix with ETFE copolymer (one virgin and one containing 0.3% BaS-10). The results are shown in Table 13.

                  TABLE 13                                                         ______________________________________                                                       No Added                                                                               Added Nucleant                                                         Nucleant                                                                               1500 ppm BS-10                                           ______________________________________                                         Solid AWG-22 Dia (mm)                                                                          0.635     0.635                                                C (pF/m)        91.9      74.2                                                 Stability of C (pF/m)                                                                          ±3.3   ±0.7                                              Foam Diameter (mm)                                                                             2.39      2.54                                                 D C             2.0       1.85                                                 % V             30        35                                                   AFCD (M)        230       38                                                   cc/cm.sup.3 at 33% voids                                                                       38,000    13,000,000                                           ______________________________________                                    

As seen from the results, this nucleating agent is very effective in this system.

EXAMPLE 10

Chemical-blown ETFE copolymer compositions were prepared by dry blending powdered ETFE copolymer with Ficel® and the sulfonic acid salt indicated in Table 14. The blends were pelletized in a lab scale California Pellet Mill before evaluation. The compositions were foamed onto wire according to conditions given in Table 3, Column E. The results are given in Table 14. In each case the amount of blowing agent was 0.38 wt. %.

                  TABLE 14                                                         ______________________________________                                         Additive  BaS-10     CaS-10   KS-8   KS-8                                      Conc.     1000 ppm   1000 ppm 1000 ppm                                                                              3000 ppm                                  ______________________________________                                         C (pF/m)  170.6      175.5    198.4  183.7                                     Foam Diameter                                                                            1.17       1.15     1.10   1.08                                      (mm)                                                                           DC        1.9        1.9      1.9    1.75                                      % V       36         36       36     45                                        AFCD (M)  76         51       51     38                                        cc/cm.sup.3                                                                              1,500,000  6,500,000                                                                               6,500,000                                                                             18,000,000                                ______________________________________                                    

As seen from the results, these compositions give good quality foam with small cell size.

EXAMPLE 11

Blends of 98% of a broad molecular weight distribution linear high density polyethylene (0.964 g/cm³) having a melt index of 1.9 (standard method 190° C./2.16 kg) and 2% of a high pressure branched polyethylene with a 0.929 g/cm³ density and a melt index of 1.9, containing various levels of sulfonic acid salts, were prepared from 0.3% concentrates and foamed as an approximately 60% void insulation onto wire using conditions given in Table 3, Column F. The results are given in Table 15. The last column shows the capacitance and the deviation from the average foam core diameter of 2150 micrometers.

                  TABLE 15                                                         ______________________________________                                                 Conc.   AFCD                                                           Additive                                                                               (ppm)   (M)         Remarks                                            ______________________________________                                         None    0       360         (a) 55-88 pF/m; ± 150 M                         FS-10   60      229                                                                    150     165                                                                    750     63          (b) 52 ± 0.3 pF/m; ± 50 M                            1500    51                                                                     3000    51                                                             CrS-10  60      203                                                                    150     152                                                                    750     102         (b) 52 ± 0.3 pF/m; ± 50 M                            1500    51                                                                     3000    38                                                              KS-10   60      254                                                                                       (a) 55-88 pF/m; ± 100 M                                 150     254                                                                    750     203         (a) 55-88 pF/m; ± 75 M                                   1500    127                                                                                       (b) 52 ± 0.3 pF/m; ± 50 M                            3000    237                                                            BaS-10  63      102                                                                    125     89                                                                     250     76                                                                     500     68          (b) 52 ± 0.3 pF/m; ± 25 M                            1000    64                                                                     3000    64                                                             CaS-10  63      76                                                                     125     76                                                                     250     64          (b) 52 ± 0.3 pF/m; ± 25 M                            1000    51                                                             KS-8    500     51                                                                     1000    43                                                                     2000    33          (b) 52 ± 0.3 pF/m; ± 25 M                            3000    33                                                             KS-6A   500     89                                                                     1000    89                                                                     2000    89          (b) 52 ± 0.3 pF/m; ± 25 M                            3000    51                                                             LS-10   63      152                                                                    125     178                                                                    250     127         (b) 52 ± 0.3 pF/m; ± 101 M                           500     101                                                                    1000    127                                                            KS-8C   500     100                                                                    1000    75                                                                     2000    64          (b) 52 ± 0.3 pF/m; ± 25 M                            3000    40                                                             NTBP    500     127                                                                    1000    76                                                                     1500    25          (b) 56 ± 0.3 pF/m; ± 25 M                            3000    76                                                             NaS-1   500     76          (b) 56 ± 0.3 pF/m; ± 25 M                            1000    64          (b)                                                        3000    51          (b)                                                KS-1    500     203                                                                    1000    178         (a) 52-88 pF/m; ± 75 M                                  3000    152                                                            KS 1(H) 500     330         (a) 69-85 pF/m: ± 50 M                                  1000    152                                                                    2000    89          (b) 56 ± 0.3 pF/m; ± 25 M                            3000    64                                                             ______________________________________                                          (a) unstable capacitance                                                       (b) stable capacitance                                                   

As seen from these results, all of these salts act as nucleants, with the KS-8 and KS-8C being especially effective. Of the S-10 salts, iron and calcium are very effective nucleants.

Comparative Example B

Several other types of organic acid salts were incoporated into the same polyethylene blend as specified in Example 11 and foamed onto wire using the conditions of Example 11. The results are shown in Table 16. The last column shows the capacitance and the deviation from the average foam core diameter of 2150 micrometers.

                  TABLE 16                                                         ______________________________________                                                         AFCD                                                                   Conc.   (% V = 60)                                                     Additive                                                                               (ppm)   (M)        Remarks                                             ______________________________________                                         None    --      475                                                            NS-12(H)                                                                               63      508                                                                    250     300                                                                    500     254          (b) 56 to 75 pF/m; ± 152 M                             1000    406                                                                    2000    457                                                            CC-18(H)                                                                               500     457                                                                    1000    457          (b) 56 to 75 pF/m; ± 152 M                             2000    457                                                             BS-12(H)                                                                               600     230                                                                                        (b) 52 to 76 pF/m; ± 100 M                             1200    230                                                                    1800    230          (a)                                                       2940    230          (a)                                               NS-12(H)                                                                               600     300          (c)                                                       1200    380          (b) like BS-12(H)                                 Na TCA  1000    304                                                                    2000    280          (b) like BS-12(H)                                         3000    254                                                            ______________________________________                                          (a) Partial plugging of the gas injector probe                                 (b) unstable extrusion                                                         (c) fairly stable                                                        

As seen from the above results, these specific salts are not effective as nucleants for polyethylene.

EXAMPLE 12

A broad molecular weight distribution, high density polyethylene copolymer (density=0.945 g/cc, melt index=0.4, standard method 190° C./2.16 kg) containing 0.75% by weight azodicarbonamide blowing agent was foamed onto wire using the conditions given in Table 3, Column G. This was compared with two, similarly chemically-blown, blended polyethylene resins, one with BaS-10 added and one without. As seen from Table 17, the BaS-10 is a very effective nucleant for this chemically blown polyethylene system.

                  TABLE 17                                                         ______________________________________                                                                          PE Blend                                                   High      PE        plus 1250 ppm                                 Properties   Density PE                                                                               Blend     BaS-10                                        ______________________________________                                         C (pF/m)     6.52      6.04      5.64                                          Foam Diameter (mm)                                                                          2.08      2.23      2.28                                          DC           1.77      1.72      1.63                                          % V          33        38        44                                            AFCD (M)     34        41        34                                            Cell Nature  (a)       (b)       (b)                                           cc cm.sup.3  16,500,000                                                                               11,500,000                                                                               22,000,000                                    ______________________________________                                          (a) irregular and interconnecting cells                                        (b) spherical and distinct separate cells                                

EXAMPLE 13

Polyarylate of the following composition was foamed according to the conditions given in Table 3, Column H.

polyarylate

2000 ppm BaS-10 powder (dry blend)

1250 ppm Expandex® chemical blowing agent

The resultant foamed composition comprised a 0.16 mm wire coating on AWG-26 solid copper wire. This coating was an excellent foam containing 30% voids with foam cells of 17 micrometers.

EXAMPLE 14

A nylon blend was foamed according to the conditions given in Table 3, Column I.

Zytel® 301 HS, nylon resin (commercially available)

5000 ppm KS-8, fully compounded

The comparison which was made in extrusion foaming, with and without KS-8 as the nucleating agent, is shown in Table 18.

                  TABLE 18                                                         ______________________________________                                                                 AFCD                                                   Resin        % V        (M)     cc/cm.sup.3                                    ______________________________________                                         Zytel ® 301HS                                                                            3         762       60                                           Zytel ® 301HS                                                                           65         508     7,800                                          5000 ppm                                                                       KS-8                                                                           ______________________________________                                    

EXAMPLE 15

A melt processible rubber (thermoplastic elastomer) having a dielectric constant of 10.2 was foamed according to the conditions in Table 3, Column J, producing the results, with and without KS-8 as the nucleating agent, shown in Table 19.

                  TABLE 19                                                         ______________________________________                                         KS-8 nucleant, ppm                                                                              none        5,000                                             % V              ˜25   ˜40                                         AFCD (M)         457         89                                                cc/cm.sup.3      4.5 × 10.sup.3                                                                       1 × 10.sup.6                                ______________________________________                                    

The aforesaid description, including the examples, and the claims which follow are intended as a disclosure of the invention and preferred embodiments thereof. There is no intent to limit the invention to only those embodiments disclosed, since one skilled in the art will readily envision other embodiments from the teaching provided herein. 

We claim:
 1. In an improved process for producing a foam of a thermoplastic resin composition, the improvement characterized in that the process is carried out with an effective amount of the nucleating agent of the formula

    [Z(CF.sub.2).sub.x (CF.sub.2 CFX).sub.p (R').sub.y (CH.sub.2).sub.z RO.sub.3 ].sub.n M

wherein: Z is CCl₃, CCl₂ H, H, F, Cl or Br; each X, independently, is selected from H, F or Cl; R is sulfur or phosphorus; M is H or a metallic, ammonium, substituted ammonium or quaternary ammonium cation; x is an integer and is 0 to 10; p is an integer and is 0 to 6; y is 0 or 1; z is an integer and is 0 to 10; x+y+z+p is a positive integer or, if x+y+z+p is 0, Z is CCl₃ or CCl₂ H; n is the valence of M; and R' is a C₁₋₁₆ straight or branched chain perfluoroalkylene diradical containing a perfluorinated alicyclic ring; a C₁₋₁₆ perfluorinated aliphatic polyether diradical with repeat units selected from [CF₂ O], [CF₂ CF₂ O] and [CF₂ CF(CF₃)O]; or, when z is 0, an aromatic diradical;so as to produce a foam having uniformity of cell size, small cell size, and high void content.
 2. Process of claim 1 wherein the thermoplastic resin is a tetrafluoroethylene/hexafluoropropylene copolymer, the average cell size is less than 100 micrometers, and the void content is greater than 60%.
 3. Process of claim 1 wherein the thermoplastic resin is substantially a polyethylene of high density, the average cell size is less than 100 micrometers, and the void content is greater than 50%.
 4. Process of claim 1 wherein the thermoplastic resin is an ethylene/tetrafluoroethylene copolymer, the average cell size is less than 100 micrometers, and the void content is greater than 50%.
 5. Process of claim 4 wherein the thermoplastic resin is an ethylene/tetrafluoroethylene/perfluorobutyl ethylene copolymer. 